The term individual differences refers to the differences in behavioral responses that are consistent over time and between contexts. Its study attempts to identify these differences and explain the factors that predisposed the individuals of a particular species to respond in a specific way. The aims of the present work were: to identify behavioral characteristics related to aggression through a correlation analysis of the C-BARQ's factors; to evaluate the effect of neonatal handling, the age and litter on the individual differences in the physiological response to stress in puppies; to identify individual differences in the behavioral response to stress in puppies and to evaluate and investigate the main features of cases of canine aggression towards family members in a referral practice and to determine the causal factors related to this problem. In order to reach the first objective we analyzed the correlations between the behavioural characteristics related to aggression and other C-BARQ's factors. A possible relation between canine aggression towards the family members and anxiety was suggested. Also, aggression towards unknown people was found to be correlated to fear. In addition, we observed a correlation between predatory behavior and aggression towards unknown people and dogs. To attain the second and third objectives, we used different litters of puppies. Each puppy was randomly assigned to one of three experimental groups: control group (non-handled), handled once a day and handled three times a day. Handling was performed from the 3rd until the 21st day of life. To evaluate the possible differences between handled and non handled animals, we evaluate the stress response by means of an Open Field Test. No differences were found between handled and non handled puppies neither in the physiological variables cortisol, rectal temperature and serum serotonin nor in the behavioral variables. We did find differences between litters in the physiological variables and individual differences in behavior. Also, serum serotonin was found to decline with age. The fourth objective was achieved by comparing clinical cases of dogs presenting aggression towards family members with control cases. We found that dogs adopted after 12 weeks of age presented a lower risk of being aggressive towards household members. Being fed from the table was also considered as a potential causal factor of dogs showing aggression towards family members. In addition, we observed that dogs presenting an underlying painful condition had a higher risk of being aggressive towards family members and were more likely to be impulsive during the aggressive events.